p2p: API cleanup and PoC 7 compatibility

Whoa, one more big commit. I didn't manage to untangle the
changes while working towards compatibility.

p2p/testlog_test.go

@@ -0,0 +1,28 @@
+package p2p
+
+import (
+	"testing"
+
+	"github.com/ethereum/go-ethereum/logger"
+)
+
+type testLogger struct{ t *testing.T }
+
+func testlog(t *testing.T) testLogger {
+	logger.Reset()
+	l := testLogger{t}
+	logger.AddLogSystem(l)
+	return l
+}
+
+func (testLogger) GetLogLevel() logger.LogLevel { return logger.DebugLevel }
+func (testLogger) SetLogLevel(logger.LogLevel)  {}
+
+func (l testLogger) LogPrint(level logger.LogLevel, msg string) {
+	l.t.Logf("%s", msg)
+}
+
+func (testLogger) detach() {
+	logger.Flush()
+	logger.Reset()
+}